Researchers supported by the National
Science Foundation (NSF) have reported the discovery of organisms that
form a protective armor of nearly perfect crystals from the atoms on the
surface of a silicon or germanium semiconductor. This characteristic
could be exploited to make faster, more stable biochips for use in the
next generation of information technology, the researchers believe.

"Instead of putting cells 'on' a chip,
this research indicates they can be put 'in' a chip, potentially reducing
the steps needed to manufacture and operate bio-based electronic components,"
said Robert Baier of the NSF Center for Biosurfaces at State University
of New York (SUNY) at Buffalo.

Scientists and engineers at two other
NSF research centers participated in the research: the Center for Microcontamination
Control at the University of Arizona and Rensselaer Polytechnic Institute
in New York and the Center for Environmentally Benign Semiconductor Manufacturing
at the University of Arizona. The Queen's University of Belfast, Northern
Ireland, also supported the research. [Amber Jones]

Background:There is a class of materials known
as semiconductors which furnish an occasional free electron for carrying
current. Silicon and germanium are the most familiar examples; they have
about one free electron for every thousand atoms (as contrasted with copper,
which has one for every atom). These semiconductors, in modern integrated
circuits, have long possessed a special interest for biomaterials researchers,
in their promise as processors of signals from biological stimuli. The
important fact about them is that the number of current-carrying electrons
and "holes" in them can be controlled or modulated by the types of single-electron
transfers common in biological processes such as respiration and photosynthesis.
Semiconductors can be made to act as conductors under some conditions and
as insulators under others. Indeed, they are so sensitive that the current
flowing in a small crystal of germanium or silicon might be controlled
by the chlorophyll in a single cell, as light shines on it in a region
where a fine wire can carry off the amplified electron "signal". This new
class of biomaterials ("biochips") can be adapted to many uses, but at
present has the sophistication of only the rude crystal detector, as used
in early radios and in an improved form in radar sets.

Mechanism of Action:Each atom in a crystal of germanium
(or silicon), has four electrons in its outer shell -- so called valence
electrons that help keep the atoms together. Because the electrons are
fully occupied in forming bonds between the atoms, they are not available
for conventional electrical flow. If some impurity which has five valence
electrons, say an atom of phosphorus from a biological reaction, gets into
the crystal, four of these electrons can form bonds with adjacent germanium
atoms, but the fifth electron will remain free to flow as electrical current.
Intimate contact of a living cell with a semiconductor can be a method
of controlling the movement and directions of electrons in the solid crystal,
by providing metabolic "sources" and "sinks" for electrons and "holes"
that are amplified in common transistor-type circuits.

Viability of the living cells is favored
since the "biochip" transistor does not need to heat up, as a vacuum tube
does, and it responds instantly. It can operate on a tiny amount of power
-- about one tenth of that used by an ordinary flashlight bulb -- and be
locally modulated by biomembrane potentials which can approach 1,000,000
volts/cm. And "biochips" can be made almost vanishingly small. The present
experimental crystal adducts of germanium and bacteria produced by us are
only about 5 micrometers on a side.

Reduction to Practice:We are exploiting the superior conductivity
of the surface layer of germanium, accounted for chiefly by the presence
of "holes" (absence of electrons). These "holes" are produced by biologically
contributed "impurities" and cell-modulated surface states, to be amplified
by the current passing through the crystal.

Our present studies show a method for
fabrication of the crystal/biology adducts and illustrate a biological
method of controlling the electrons or "holes" in a crystal. Similar to
the approach which led Bardeen and Brattain to the invention of the original
transistor, the initial device we envision also consists of two fine conductive
wires of which the tips, only a few micrometers apart, rest on a germanium
+ cell adduct crystal conductively bonded to a metal disk. These elements
will be housed in a metal cylinder which is connected electrically to the
metal disk and crystal, thus forming the ground terminal. The "cat's whisker"
wires will be connected to pins that can be plugged into a socket.

So far, fundamental new knowledge has
been gained about how biological cells may be incorporated into and modify
the structure and energy states of solid matter and the electrical behavior
of the surface atoms in a semiconductor. Basic study of these phenomena
continues.

Prospects for Manufacturing:Industry experts have declared many
times that some physical limit exists below which miniaturization of integrated
circuits could not go. An equal number of times they have been confounded
by the facts of functioning smaller, less expensive devices. No limit can
be discerned in the quantity of transistors that can be fabricated on germanium
or silicon, which has increased through eight orders of magnitude in the
50 years since the transistor was invented.

It is anticipated that "biochip" semiconducting
materials, based on germanium and silicon, can be made into functional
transistors in an integrated process involving many of the steps now employed
for integrated circuits. Templates, called masks, will be applied to the
"bio-doped" germanium or silicon in order to expose desired areas. Next,
various operations involving chemical diffusion, radiation, doping, sputtering
or the deposition of metal can act on these areas, sometimes to construct
device features, other times to erect scaffolding to be used in succeeding
steps and then removed. Meanwhile, other devices -- resistors, capacitors
and conductors -- can be built into the same circuit to connect the "biochip"
transistors.

Extensions to Biophotonics:One major difficulty that continues
to limit the development of photonics, and especially nonlinear optical
devices, is that the intensities of difficult-to-amplify optical beams
replace the easily boosted currents and voltages of electrical circuits.
The photonic operations depend on fine-tuning the system so that a small
input will upset a delicate balance. Although such switches have been called
"optical transistors", they do not share the amplifying principles of transistor
action.

"Biochip" optical switches, in a biophotonics
circuit, can overcome this fundamental problem. Although light hardly interacts
with light, the interaction of light-induced signals is essential for photonic
functions. With "biochips", optical signals can be converted by integral
embedded cells into electrical ones in a semiconductor. The voltage thus
produced will change the optical response of another material, thereby
modulating a second beam of light.

Another use of the semiconductor-to-biology
symbiotic combinations may be to produce light-sensitive "heterojunctions",
in which crystalline lattices of different energy gaps meet. "Bio-doped"
and conventional crystalline lattices will mesh imperfectly, creating atomic-scale
defects, or elongate toward one another, creating elastic strain. Either
defects or strain can produce useful biophotonic electrical side effects
that can drive circuitry.

These combinations will involve complicated
biophysics but at the same time will provide a tunable variable that will
be useful in surmounting many design problems.